There is no one standard answer. The maximum number of hours that a student should work in year 11 depends on their own circumstances. Year 11 may offer flexibility with work hours given that none of your year 11 results will contribute towards your HSC results or your ATAR (assuming that you are not accelerating any subjects). This would allow you to work more hours if you feel that you can/should. At the same time, however, many students use year 11 as a period to test and optimise their study strategy at no expense (as opposed to having to do that in year 12, when everything counts). This could affect you depending on your perception of year 11. If you are planning to make the most of year 11, then perhaps 15 hours should be the maximum, i.e. perhaps you should not work for more than 15 hours each week. However, if you plan to leave the hard work till year 12, then perhaps there are less restrictions on the number of hours that you can work each week.
Much of the same applies to year 12, although you must be careful with time management given its more sensitive nature. I personally would not recommend working during year 12 unless there is a need to (especially if your goal is to maximise your ATAR, which would likely require more study time). I think that university provides more flexibility, allowing you to work throughout your university years instead of year 12. Of course, numerous students have worked in year 12 and still achieved favourable results, which is the reason that such a decision depends on your own circumstances.
I hope this helps!